Wednesday, September 18, 2013

Variable speed fans in Packaged Units over 10 tons

Variable speed fans in Packaged Units over 10
tons

On a recent job I specified two 10-ton rooftop
units. The application was a Class 1000
Clean Room. I wanted constant volume rooftop AC units—initially, air handling
units were discussed but because of budget constraints we had to settle for
packaged units. Later, when I reviewed
the submittal that came from the contractor I did see the wording ‘Multi speed
fan’ but did not get alarmed as many manufacturers provide units with low,
medium, and high speed fan settings that is just a one-time adjustment at the
start-up. It turned out that because of
the latest Title 24 mandates, all AC units over 10 tons now need to have
varying speed fans. The fan speed is varied
based on the staging of the compressors.
In the old days, a ten ton unit will supply, say, 4000 CFM all the time,
and the supply air temperature will be changing based on the staging
compressors, but now the supply CFM will vary based on the staging of the
compressors and the supply air temperature will remain almost constant.

This Department of Energy documents describes
the changes meant to increase the efficiency of rooftop units:

In short, the AC units we received had variable
speed fan motors. Now the challenge was
to change the fan to constant speed. The
first set of instructions from the manufacturer on changing the fan speed to
constant did not work. These were the
instructions.

>>Unplug the J9 off the RTRM then unplug
the J8 pins 6&7. Reset power to unit
and adjust the R136 to the desired fan speed.

The contractor kept talking to the engineering
department of the manufacturer. The
second set of instructions did work.
Here is what worked:

I wish AC unit manufacturers will say it very
clearly on their brochures: The supply fan speed is automatically varied based
on the cooling requirements. This is the
default unit that will be shipped to you unless you specifically ask for a ‘Constant
Speed Fan’ option.